What is CVE-2025-43859?

CVE-2025-43859 is a critical-severity vulnerability in Python-Hyper H11, classified under Inconsistent Interpretation of HTTP Requests (HTTP Request/Response Smuggling). CVSS score: 9.1/10. Published 2025-04-24.

How severe is CVE-2025-43859?

Critical severity. CVSS v3 base score is 9.1 out of 10.

Is CVE-2025-43859 known to be exploited?

8 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.

Vulnerability in Python-Hyper H11

CVE-2025-43859

h11 is a Python implementation of HTTP/1.1. Prior to version 0.16.0, a leniency in h11's parsing of line terminators in chunked-coding message bodies can lead to request smuggling vulnerabilities under certain conditions. This issue has been patched in version 0.16.0. Since exploitation requires the combination of buggy h11 with a buggy (reverse) proxy, fixing either component is sufficient to mitigate this issue.

Vulnerability class: HTTP Request Smuggling

EPSS: 0.006 (44.3th percentile) — read the EPSS interpretation.

CVSS v3 metric

CVSS v3 base score 9.1 (Critical). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N.

Affected products

Python-hyper H11 — versions < 0.16.0

Weakness classification (CWE)

CWE-444 — Inconsistent Interpretation of HTTP Requests (HTTP Request/Response Smuggling)

Public proof-of-concept exploits

References

security-advisories@github.com (x_refsource_CONFIRM)
security-advisories@github.com (x_refsource_MISC)

Frequently asked questions

What is CVE-2025-43859?: CVE-2025-43859 is a critical-severity vulnerability in Python-Hyper H11, classified under Inconsistent Interpretation of HTTP Requests (HTTP Request/Response Smuggling). CVSS score: 9.1/10. Published 2025-04-24.
How severe is CVE-2025-43859?: Critical severity. CVSS v3 base score is 9.1 out of 10.
Is CVE-2025-43859 known to be exploited?: 8 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.